Sex differences in human brain and cognition are well documented, but it remains unclear whether men and women follow different trajectories of age-related cognitive decline. Identifying predictors of differential cognitive aging between men and women could lead to the development of sex-specific interventions to promote successful cognitive aging in both sexes. Because human cognition is strongly influenced by socio- cultural and environmental factors, studies in nonhuman primate models, for which such confounds can be minimized, can provide considerable insight into the biological bases of sex differences in cognitive aging. This application uses a translational primate model uniquely suited for longitudinal assessments of neurocognitive aging, the common marmoset (Callithrix jacchus). With a lifespan of about 8 years, marmosets can be studied throughout their entire adult lifespan. In addition, they are able to perform complex cognitive tasks and experience age-related brain changes that resemble those of humans. This project will study intact male and female marmosets longitudinally from middle-age to old age. Sex differences and age-related changes will be assessed in monkeys tested annually for four years on a battery of cognitive, emotional and motor tests. Sex differences and age-related changes in brain structure and functional connectivity will also be examined annually using Magnetic Resonance Imaging (MRI) and resting- state functional MRI (fMRI), respectively. Sex differences in dendritic spine density and morphology, hippocampal estradiol levels and -amyloid deposition will be analyzed in the brains of aged monkeys characterized as cognitively impaired or unimpaired. This study should provide invaluable information on the biological correlates of sex and age differences in cognition in a translational primate model followed from middle-age to old age. The data will serve as a fundamental foundation for understanding the role of sex hormones in neurocognitive aging. Ultimately, the project should promote sex-specific approaches to the management of cognitive impairment in older people.